Measuring the topography of high-curvature surfaces associated with various objects has in many occasions turned out difficult. Traditional optical methods are limited to flat surfaces. For small and flat surfaces e.g. interferometers can be used, but they are expensive, slow and provide unacceptable accuracy.
Different methods incorporating physical contact with the target objects are also often tedious, provide inferior horizontal resolution and even scratch or otherwise damage the potentially delicate surface under analysis. Such drawbacks are rather comprehensible in the light of point-by-point scanning methods typically applied. Alternative machine vision—based arrangements do not perform too well either, particularly in connection with glossy surfaces.
Shape measurement of high-curvature glossy objects is indeed generally demanding to say the least. Currently e.g. the quality control of glossy objects is made manually using visual inspection in bright illumination. The outcome of such inspection depends highly of the expertise of the particular human inspector in charge and varies also with time and the manufacturing process itself. Using manual inspection only rather vague qualitative results can be obtained; more characterizing, or at least more exact, numerical values representative of the defects or 3d shape remain practically secret. However, many glossy products are considered as high quality or ‘high end’ products, whereupon even small defects should be preferably identified during or briefly after the manufacture thereof.
One optical method for tracking glossy surfaces is based on the use of a flat digital display (e.g. TFT-monitor) and a camera. The display may be arranged to show fringe patterns and the camera may observe patterns as reflected via the tested surface. Analysis regarding the phase changes of the pattern between original and reflected version may then reveal the slope of the surface with reasonable accuracy and overall execution time, but the applicable maximum curvature of the object stays rather limited, i.e. the object shall be still be relatively flat for obtaining comprehensive analysis results of decent accuracy.